This is actually about the concrete usage of "了". Because if you describe sb. as a mad man, it's usually under the condition or assumption that this man is already mad before you say he is mad. That's why there is usually a "了" attached to the "疯". That's the usage of "了" here: to have transformed into a condition". "了" is not equal to past tense, many English native speakers will be influenced by habitual thinking of their mother tongue when they use this character. To say "你疯了" is exactly to imply "I think you are already mad before I say you are mad" rather than "You were mad". It's kind of like present perfect tense. So, as you can conjecture, "了" is optional here, and this rule can also be applied to other similar conditions. e.g. if you think someone is going to be crazy, you can say "我觉得他可能要疯". And "疯" can combine other characters to make words, like "疯子", "疯狂的", "疯疯癫癫" etc.

we can also say"疯子；疯狗" and something like that. If 疯 was used with 了 together，it means a passed status.It can also be used with nouns

It's because in most cases what you'd be asking is the rough equivalent of "have you gone mad?" rather than "are you insane?" that is to say, your friend was fine before, but after what he's said youre sure he's become crazy.